Fatigue and/or crack growth test sample

ABSTRACT

A sample for fatigue and/or crack growth testing, including an axisymmetric or cylindrical gage section with a concentric hole running from a first end, and terminating within the gage section, with one mode of loading introduced at the terminus of the hole, and reacted at the end where the hole originates. A second mode of loading is optionally introduced at a second end of the specimen. Use of the specimen is described in both in the context of an apparatus for fatigue/crack growth testing described in the referenced parent application, as well as with conventional test machines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 13/031,410 filed 2011 Feb. 21now U.S. Pat. No. 8,544,338, granted Oct. 1, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to an novel sample configuration for performingfatigue and/or crack growth tests including complex loading with regardto the relative magnitude and waveform of the load cycles, with theability to apply axial tension, compression, and/or torsional loadingindependently, potentially resulting in fully mixed-mode crack growthwith non-proportional loading. While resonant (dynamic) conditions maybe possible to achieve with the sample, an important object of theinvention is to extend the advantages of closed-loop, non-dynamictesting to moderately high frequency ranges. Another object associatedwith crack growth applications is to provide a crack growth test optionwith a uniform constraint and/or plasticity-induced closure state acrossthe crack front. Further objects include the ability to configure such aspecimen with a stress-intensity that reduces as the crack lengthincreases during a constant load or constant load amplitude test, and toachieve a well-defined, stable crack shape during crack growth. Theseand other objects, advantages and characteristic features of the presentinvention will become more apparent upon consideration of the followingdescription thereof when taken in connection with the accompanyingdrawings depicting the same.

2. Description of the Prior Art

Fatigue and/or crack growth testing is necessary in many engineeringapplications where component durability and safety concerns merit theassociated costs. For cyclic testing, it is often desirable to increasethe frequency of such testing to more closely simulate field conditions,as is particularly true for high-cycle fatigue or crack growth thresholdtesting. Nevertheless, high frequency testing is also desirable if forno other reason than to reduce the duration and cost of testing. Themost common fatigue test machines apply cyclic load to a sample mountedbetween two connection points with cyclic loading supplied byservohydraulic, or servoelectric actuation systems, and seldom exceed100 Hz frequency capability due to inherent design limitations. However,it is not uncommon for these machines to employ closed loop load,displacement, or even crack tip stress-intensity control capable ofarbitrary load waveforms and complex loading sequences, which can bevery desirable in some applications. The use of these types of machinesand the common samples employed for fatigue and crack growth testing aredescribed by ASTM standards (especially ASTM E466 and E647) and is wellknown to those familiar with the art.

Application Ser. No. 13/031,410 pertaining to a device for cyclicloading of a test sample, is related to the present application asreferenced above, and provides an extensive description of prior arttest devices relevant to achieving higher test frequencies. While thesedevices are not specifically relevant to the present application for atest sample, it is largely true for cyclic loading devices that thehigher the stiffness of the sample, the higher the possible operationfrequency, particularly for non-resonant conditions involvingclosed-loop, arbitrary waveform operation. While treatment of thestiffness of the test machine and its elements is not uncommon in theart, the stiffness of the specimen is a less common design object. Thespecimen configuration associated with an ultrasonic piezoelectricdynamic system operating at 15-30 MHz as described in Gigacycle Fatiguein Mechanical Practice, by Paul C. Paris and Claude Bathias CRC Press2004, is one exception, associated with a specific resonant testingsystem, and resembling a common “dogbone” style specimen.

With regard to prior art in sample geometries for fatigue and crackgrowth testing, the most commonly used configurations are described inthe ASTM standards referenced previously, with several other potentialtest configurations described in stress intensity handbooks such as theStress Analysis of Cracks Handbook, 3^(rd) Ed (H. Tada et al, ASMEpress, 1997). The compact tension specimen is of particularly commonusage for crack growth, but is well known to have reduced constraint inthe vicinity of the intersection between the crack front and the freesurface, resulting in non-uniform plasticity induced closure across thecrack front for cyclic applications. Specimens with quarter circular orsemicircular cracks are also popular, and have the benefit of resemblingcommon naturally occurring crack shapes, but are also subject to freesurface effects, though to a lesser degree. Free surface effects areabsent in samples with a fully circular crack front, such as circularcylindrical or tubular specimens with a circumferential crack loaded intension. However, because the stress intensity increases with cracklength for these configurations (for a given load), any deviation from atruly concentric crack front creates an uneven stress intensity, withthe highest stress intensity where the local crack length is longest.Thus the crack shape tends to become more irregular as the crack grows,resulting in crack front shape instability. This hampers correlation ofthe data with a single standard stress intensity solution, and impairsthe reproducibility of results. The short rod chevron notched specimendescribed in U.S. Pat. No. 4,116,049, is one of the few specimens knownto have a reducing stress intensity solution, which is advantageous forits use as a fracture toughness specimen, but the shape of the crackfront, which is generally assumed to be straight for analysis purposestypically exhibits significant curvature. In fact, nearly all commonlyused crack growth sample configurations exhibit crack shapes thattypically differ from those assumed in the stress intensity solutions,introducing a degree of error in the interpretation of the results. Thisdeficiency is not easily corrected merely by a more careful analysis ofthe specimen, because it is linked to free surface effects and can bematerial dependent.

SUMMARY OF THE INVENTION

The invention pertains to a sample, or specimen for performing fatigueand/or crack growth tests, that is suitable for testing in variousloading devices found in the art, but which is particularly well suitedfor testing in a device for applying cyclic loads which is the topic ofapplication Ser. No. 13/031,410 referenced above, and which will be usedas an exemplary context for the description of the features and benefitsof the sample. In order to offer high frequency cyclic capability, thetest device employs at least one actuator based on a solid statematerial system which undergoes deformations in response to theapplication of energy, such as a piezoceramic material operating undercyclic electrical input. Such materials can be oriented and energized insuch a manner as to produce axial or shear (torsional) deflections, butgenerally are capable of relatively small strains for use in actuation.In order to enable high-load/high-frequency operation without thelimitations associated with resonant operation, the full load rangeprovided to the specimen must be supplied by the actuator (withoutdynamic amplification). This requires that the total load trainincluding the test device and the sample connected together must be asstiff as is practical to maximize the load capability for a givenactuator. This also allows the apparatus to retain the capability toperform more conventional low frequency testing where that is arequirement.

Additionally, solid state actuators are typically weak in tension, buttesting in tension is a common necessity. Thus the test sample of thepresent invention, in embodiments that involve axial actuation, isnaturally configured so that when operated with the device of referencedapplication Ser. No. 13/031,410, an axial compression load in theactuator results in an axial load of the desired sign in the specimen,even if the specimen is to be loaded in tension. Further, protectivecompressive preloading is applied to the axial actuators by preloadingthe specimen directly (in tension) without any parasitic load beingdiverted to a separate prestressing member, nor is any such member inthe tensile load path to reduce the load train stiffness. This alsoenables the practical application of a substantial amount of preload,enabling the use of bipolar excitation of the actuator, which increasesthe load range capability.

For a single loading mode, such as axial loading, the device ofreferenced application Ser. No. 13/031,410 includes two load frames, aninternal load frame substantially enclosed by a first external loadframe, with at least one guiding interface between them to maintain asubstantially concentric and coaxial alignment during operation. Thisarrangement provides the stiffest practical means to transmit a loadfrom an axially deflecting actuator system to a sample and back again,completing a load circuit. The sample is connected to a first end of theinternal load frame and first end of the first external load frame,forming a first load path from the internal load frame through thesample to the external load frame. Cyclic loads are transmitted to thesample from the actuator or system of actuators which extends along theaxis between a second end of the internal load frame and a second end ofthe external load frame, imparting cyclic load via the first load paththrough the sample.

It should be noted that the use of the word “end” herein should not bestrictly limited to denote only the very extreme extent of the oppositesides of a component in the strictest sense, but in a more general sensewherein the two ends denote two regions of a component substantiallyincluding the opposing extremities with an intermediate region between.Reference to interfaces or connections at the ends so describedpresupposes suitable interfacing features or means of attachment.

The guiding interface spoken of may be embodied as a region of directcontact between adjacent load frames, or indirectly as an interfacebetween one of the load frames and the sample, which is rigidly attachedto the other load frame, or both, so long as the alignment of allmembers of the load frames is substantially maintained thereby duringoperation.

The load frames, though described as a single component, may consist ofmultiple members, jointed by force of direct contact or other meanssuitable for the type of load being transferred. The load frames mayinclude sensing hardware, such as a load cell for instrumentationpurposes.

As mentioned before, it is desirable to prestress the actuators incompression, particularly axial actuators, both to protect the actuatorfrom damage, and also to facilitate the running of tests with a highmean load, and potentially to take advantage of the increased load rangeand actuator durability achievable by bipolar operation for someactuator types. Preloading is achieved with the sample mounted byproviding an adjustable length connection at some point within the firstexternal load frame or its connection either to the actuator or thesample, by which the complete load train representing the device and thespecimen can be tightened to put axial actuators in compression when inthe neutral state (power off). Care should be taken in the design of theadjustable length connection to make it sufficiently stiff. For example,if a threaded connection is used, the optional addition of a locknutwill result in a stiffer connection, improving the load capability ofthe machine. Similar measures should also be considered at any otherdetachable connections in the device.

For axial loading, the sign of the loading is defined herein by the signof the loading in the first internal load frame, which is typically thesame as the sign of the predominate stress in the specimen. To run thedevice in tension, the actuator must be oriented so that when it is incompression, the first internal load frame is in tension. The actuatorand first external load frames experience stress of opposite sign inthis preferred arrangement.

For a device configured for axial tension loading, the internal loadframe experiences stress substantially of the opposite sign from thefirst external load frame, and is thus in compression, like theactuator. To avoid torsional or bending loads in the actuator as aprotective measure, the internal load frame, or the first external loadframe adjacent to the attachment to the actuator at its second end, maypreferably include two members, which transfer load by direct contactbetween a locally spherical convex surface in one member, and a either aflat surface, or preferably a matching concave spherical surface in theother member. The interface between matching spherical surfaces ispreferred for most applications because it provides a stiffer connectionaxially than a spherical/flat interface. The amount of torsional loadtransferred by friction can be kept small by limiting the diameter ofthe contact area. This may be particularly useful if the adjustablelength connection described above for prestressing the actuator is athreaded connection.

To enhance the stiffness of the apparatus, it is beneficial to utilizehigh modulus materials in the load frames, such as a form of tungstencarbide, which can exhibit an elastic modulus up to 90,000 ksi orhigher. For elevated temperature testing, it is required to insert themounted sample into a furnace during operation. To avoid overheating ofthe actuators or load cells one or more members of the various loadframes may consist of a material of low thermal conductivity. Zirconiaceramic, which has low thermal conductivity, but also high strength andhigh elastic modulus (about 30,000 ksi), is particularly well suited forthis purpose.

Active cooling may also be necessary both to alleviate furnace heat orheat generated within the actuators during operation. This can beaccomplished by utilizing cooling passages through the load frames, andespecially in gaps left for this purpose between adjacent load frames,or between actuators and load frames, thus allowing these members toserve as cooling channels as well as structural members.

As mentioned above, instead of a single actuator, actuator systems maybe driven by more complex actuation systems. Novel concepts for highstroke actuation systems combining the strokes of more than one actuatorin an actuation system will be described in the detailed description ofthe preferred embodiments.

Because of the high stiffness of the test device described, and theavailability of rapid response solid state energy conversion materials,and piezoceramics in particular, it is estimated that the device couldbe operated at frequencies up to 2000 Hz with sufficiently powerfulelectronics, active cooling, and with a sufficiently stiff specimen.

The performance of the test device is dependent to a large degree on thestiffness of the specimen. While the machine can be configured to testvarious specimens, a description of the novel specimen concept whichconstitutes the invention claimed herein now follows. This new specimengeometry consists of a length of the material to be tested, with a firstand second end, of axisymmetric and preferably circular cylindricalshape on the exterior over at least a portion of its length, wherein asubstantially circular hole extends from the first end of the samplealong its longitudinal axis to a depth such that its terminus lies inthe midst of the axisymmetric and preferably cylindrically shapedportion. The shape of the hole in the region of its terminus acts as anotch to initiate and grow cracks in the specimen when cyclically loadedby at least two means, a first means introducing load in the vicinity ofthe terminus of the hole, and a second means introducing load at or nearthe first end of the sample, distributing load in a substantiallyaxisymmetric manner around the sample axis, such as by a threadedconnection, or by the means of a an axisymmetric retaining flange at thefirst end of the sample. An extension of the internal load frame isguided into the hole, is preferably held in alignment thereby, andinterfaces with the hole in the vicinity of its terminus preferably bydirect contact between substantially matching flat, spherical, orconical surfaces for the purpose of applying load directed along theaxis of the sample.

This sample and loading concept is very stiff, and is well suited tohigh frequency operation in a test device such as that described herein.

Of particular interest for fatigue crack growth testing is a sampleconfiguration as described above, but more specifically comprising aflat bottomed hole with substantially sharp corners and an internal loadframe extension with a matching flat, substantially sharp corneredinterface. This configuration, when tested under cyclic loading,typically results in an annular crack emanating from the corner of theflat bottomed hole. If the ratio between the hole diameter and exteriorsample diameter in the vicinity of the hole terminus is kept below about0.6, the stress intensity factor at the crack tip for a constant (orconstant amplitude cyclic) load will reduce as the crack grows,promoting stable concentric crack growth. Further, this natural tendencyto shed stress intensity as the crack grows is advantageous in some testcircumstances. For example, a crack growing in a specimen tested atconstant load amplitude will tend to slow as it grows, potentiallyarresting as it approaches the crack growth threshold. Most conventionalspecimens have a stress intensity that increases as the crack grows,typically requiring carefully controlled load shedding to obtainthreshold results. Also, the full circular crack front exhibits uniformconstraint, and thus a uniform plasticity-induced closure state forcyclic loading, making it of special interest for the study of crackgrowth.

Based on the above discussion, samples with diameter ratios less than0.6 may be preferred for crack growth tests when a reducing stressintensity profile is advantageous for the test objective. Otherwise,conditions specific to the test objectives may influence one familiarwith the art to choose different diameter ratios or otherconfigurations. Stress intensity and stress concentration factorsrequired with regard to the use of a given configuration in testing canbe determined using finite-element or boundary element methods common tothe art.

An optional second independent mode of operation, such as torsionalloading, may be added to the test device described previously by theinclusion of a second external load frame, substantially enclosing theinterior and first external load frames over at least a portion of theircombined length, and with at least one guiding interface between thefirst and second external load frames (acting directly or indirectlythrough the sample) to maintain concentric and coaxial alignmenttherewith. This second external load frame also connects to the sampleto create a load path from the second load frame, through the sample, tothe first external load frame. Cyclic loads are transmitted thereby tothe sample from a second solid state actuator or system of actuatorswhich extends between the second end of the first external load frameand the second end of the second external load frame. This actuatorsystem includes a solid state energy conversion material oriented andenergized so as to produce deflections in the direction of the desiredloading corresponding to the mode of operation, preferably compression,or torsional loading.

The independent load frame arrangement inherently separates actuationsystems of different modes of operation so that they do not falldirectly in each other's load train, avoiding the loss of stiffness thatwould otherwise occur. For embodiments with both axial and torsionalmodes of operation, however, it is also necessary to further isolate theload trains so that actuation in one mode will not load the actuatorcorresponding to the other mode, as a protective measure, since theactuators are typically weak with regard to loads in anything but thedirection of actuation. If the two modes of operation are chosen to betensile and torsional as described in the foregoing, isolation of thetorsional stage can be achieved by introducing the torsion through amember that is stiff with regard to torsional displacement, but flexiblewith regard to axial displacement, such as a thin plate or a leafspring. The protection measures for the axial stage have already beendiscussed. Depending on the application, other means of isolation mayalso be chosen by one skilled in the art.

Novel concepts for high stroke torsional actuation systems combining thestrokes of more than one actuator in an actuation system will bedescribed in the detailed description of the preferred embodiments.

When using the preferred sample with a two-stage test device with thesecond mode of operation being compression or shear, the compressive ortorsional load is introduced at a third location at or near the secondend of the specimen (beyond the terminus of the hole).

Lastly, it is also possible to configure mounting hardware to enabletesting of the preferred sample geometry in prior art servoelectric,servohydraulic, or other types of machines, albeit subject to thelimitations of those machines. Examples of these embodiments will bedescribed in more detail hereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of example with reference toembodiments that are illustrated in the figures, but without therebyrestricting the general object of the invention. In these figures,

FIG. 1 shows a schematic representation of the design of a fatigue crackgrowth test apparatus with a single mode of operation including both thedevice for applying cyclic loads and a sample configured for crackgrowth testing.

FIG. 2 shows an exploded view of the apparatus in FIG. 1, but withoutthe schematic representation of the power source.

FIG. 3 shows an enlarged view of a sample configured for axial fatiguetesting mounted with an exemplary threaded attachment configuration on adevice otherwise identical to FIG. 1.

FIG. 4 shows a schematic representation of a fatigue crack growth testapparatus with a multi-part first external load frame, furtherconfigured for high temperature testing.

FIG. 5 shows a schematic representation of a fatigue crack growth testapparatus with independent axial tension and compression loadingcapability.

FIG. 6 shows a schematic representation of a fatigue crack growth testapparatus with independent axial tension and torsion loading capability.

FIG. 7 shows a schematic representation of a fatigue crack growth testapparatus with independent axial tension and torsion loading capabilityillustrating high-deflection solid state actuation system configurationsfor each mode.

FIG. 8 illustrates connecting hardware to mount the preferred specimenin a conventional test machine configured with loading platens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a schematic representation of an apparatus forperforming fatigue or fatigue crack growth testing, including a sample 1to be tested and a device for applying cyclic loads thereto. The cyclicloading device illustrated includes an internal load frame 2 and a firstexternal load frame 3, each having a first end and a second end, Thefirst external load frame 3 substantially encloses the internal loadframe 2 over a portion of its length, and creates a guiding interface 4therewith to maintain a substantially concentric coaxial alignmentbetween the load frames. The sample 1 having two ends and a bore holeoriginating in the first end can be attached by way of two substantiallyrigid detachable connecting interfaces 6, 7 to the first ends of thefirst external load frame 3 and internal load frame 2, respectively,forming a load path through the sample, by which cyclic loads aretransmitted through the sample to the first external load frame 3 fromthe adjacent load frame 2. The loads originate from an actuator (oractuator system) 5 which extends from the second end 9 of the firstexternal load frame 3 to the second end 8 of the adjacent load frame 2,so as to impart cyclic loading via the load path through the sample 1.The actuator 5 includes a solid state material system which undergoesdeformations in response to the application of energy, with theorientation of the solid state actuation material and the application ofenergy such that the deformations occur predominantly in the directionof the desired loading. For the purposes of illustration, it is assumedthat the solid state actuator 5 includes a preferably piezoceramicmaterial configured to deflect with either an angular or preferablyaxial displacement with the application of electricity from the cyclicpower source 11. FIG. 1 also shows a preferred arrangement of theactuator such that the predominant orientation of stress in the actuatormaterial due to an imposed deflection is of opposite sign to theorientation of the stress in the first external load frame, therebyloading the specimen in tension, when the actuator is loaded incompression. An adjustable length connection (not explicitly shown, butpossibly integrated into the connecting interface 6), allows theassembly to be tightened together to place the actuator in compressionin its neutral state.

FIG. 3 illustrates the use of a threaded connection interface 6 betweenthe first end of the first exterior load frame 3 and the first end ofthe sample 1, suitable for reacting the preferred (tensile) axial loads23 shown for illustration. The threaded arrangement also allowsadjustment of the length of the total load train created by the sample 1and the load frames 2 and 3, allowing the actuator (not shown in FIG. 3)to be preloaded in compression in the neutral state. FIG. 3 furtherillustrates a multi-component internal load frame wherein the sample 1engages an internal load frame extension 14 and wherein the internalload frame extension contacts the primary internal load frame 2 with aspherical/flat interface 32. The spherical/flat interface 32 betweeninternal load frame components protects the axial actuator (not shown inFIG. 3) from potentially damaging torsional loading resulting from thetightening of the threaded connection, as well as any loads arising frommisalignment.

FIG. 4 shows a version configuration of the apparatus configured forelevated temperature testing with axial tension loading, includingvarious enhancements. Enhancements in this illustration include anelongated sample 1, further additional components in the internal loadframe, and a multi-part first external load frame. For illustrationpurposes, the configuration of the sample 1 has been extended toprotrude into a furnace or other heat source 18, and illustrates the useof a flanged interface connection 6. A load cell 22 for testinstrumentation is included as a member of the internal load frame 2.For illustration purposes a button type load cell 22 with a convexspherical interface is depicted, mating with a concave surface in theadjacent member of the internal load frame 2. This interface providesimproved stiffness compared to the sphere-to-flat-interface of FIG. 3,but remains flexible with regard to bending and torsion, adjusting forsmall amounts of misalignment and protecting against torsionalpreloading induced during tightening of an adjustable length connection27, also shown to preload the load axial actuator 5 in compression. Asan option to provide additional stiffness, locknuts 33 are shownsecuring the adjustable length connection 27, and the specimenconnection interface 6. The load cell 22 and the actuator 5 are locatedaway from the heat source 18 as a protective measure, to avoidoverheating these potentially heat sensitive components. As a furtherprotection, insulating members 19 are included in the external loadframe 3, and between the internal load frame 2 and the internal loadframe extension 14. These insulating members require a combination oflow thermal conductivity and high modulus and strength, thus a zirconiaceramic is preferably used. As a further protection against heat fromthe both the heat source 18 as well as from internal heating within thesolid state actuator 5, air or another cooling medium is circulatedalong paths 21 in channels or gaps between the adjacent load frames 2and 3, and/or between actuator 5 and load frame 3.

In any embodiment, but particularly in elevated temperatureapplications, the compliance of the overall load train potentiallylimits the loads that can be applied for a given actuation system. It isthus advantageous to employ high modulus materials, such as a tungstencarbide, in the load frames 2, 3 and any connecting hardware, especiallyin the most compliant members, such as members of the internal loadframe extension 14.

Because of the high stiffness of the cyclic test device described, andthe availability of rapid response solid state energy conversionmaterials, and piezoceramics in particular, it is anticipated that thedevice could be operated in closed loop mode at frequencies up to 2000Hz with sufficiently powerful electronics, active cooling, and with asufficiently stiff sample, such as will now be described.

The preferred sample 1 illustrated in FIGS. 1-3, consists of a length ofthe material to be tested, of circular axisymmetric shape generally, anda preferably cylindrical shape on the exterior over at least a portionof its length, wherein a substantially circular hole extends from thecenter of a first end of the sample, along its longitudinal axis to adepth such that its terminus 16 lies in the midst of the saidaxisymmetric and preferably cylindrically shaped portion, the shape ofthe hole in the region of its terminus 16 acting as a notch to initiateand grow a crack 20 in the sample 1 when subjected to cyclic loading.The configuration shown in FIG. 1, with a sharp-cornered flat bottomedterminus 16 and matching loading interface 7, is well suited for fatiguecrack growth testing. In FIG. 3, the more rounded terminus 16 is moresuited to fatigue/crack initiation testing. Note also that the means ofconnection between the first end of the internal load frame 2 and theterminus 16 in FIG. 3 includes a separate internal load frame extension14 which makes a guiding interface 4 within the sample to maintain aconcentric and coaxial alignment with the sample.

Many variants on the shape of the hole terminus 16 and the interfacewith either the internal load frame 2 or the internal load frameextension 14 can be made by one familiar with the art to best serve theobjectives of the testing, including, but not limited to configurationswhere load is transferred by way of substantially matching flat,spherical, or conical surfaces.

As explained previously, for applications wherein the test objective isbest met with a stress intensity factor that reduces naturally as thecrack grows, the preferred sample configurations 1 may be advantageouslyconfigured to have an interior to exterior diameter ratio below 0.6 inthe section proximate to the hole terminus 16.

FIGS. 5 and 6 show schematic representations of embodiments that includean optional second external load frame 10 to introduce a second mode ofoperation. The second external load frame 10 substantially enclosesother load frames 2, 3 over at least a portion of their combined length,making a guiding interface 4 with the adjacent load frame 3, therebymaintaining a concentric and coaxial arrangement between the variousload frames 2, 3, and 10. In each illustration, the actuator 5 extendingbetween the internal load frame 2 and first external load frame 3provides tensile loading of the sample. An additional actuator 17extending between the first and second external load frames 3, 10 isconfigured to provide compressive loading in FIG. 5, and torsionalloading in FIG. 6. An embodiment of the preferred specimen is also shownwherein additional mode of loading is transferred by way of a thirdconnecting interface 13 at the second end of the sample 1. Many of theenhancements shown in prior figures are excluded here for simplicity ofillustration, but could be similarly applicable.

FIG. 6 also shows two isolating features necessary to isolate the axialactuator 5 and torsional actuator 17 stages including a sphericalcontact surface interface 32 to protect the axial actuator 5 fromtorsion, and an axially flexible member 15, such as a thin plate or aleaf spring, that protects the torsional actuator 17 from axial tension,but is sufficiently stiff to transfer torsional loads. The torsionalactuator 17 is shown to be of tubular geometry for illustrationpurposes, but could also be of other configurations.

FIG. 7 shows an enhanced tension/torsion concept illustrating multipleactuator actuation systems for tension and torsion that are designed toprovide increased deflection over single actuator systems. The axialactuation system shown has two actuators 5 and 25, but will be describedin terms of its general form, which utilizes at least two axialactuators, including one solid cylindrical actuator 5, and at least onetubular actuator 25, wherein the actuators are nested coaxially in asubstantially concentric manner, held in place by a single intermediatemember 24 with a first and second end, and wherein the odd member(s) 5(numbered from the center outward) are inserted into at least one recessin the intermediate member 24 open to the first end 28, and the evenactuator(s) 25 are inserted into the at least one recess in theintermediate member 24 open to the second end 29. The free ends of theactuators protrude from each end 28, 29 of the intermediate member andconnect to the second ends of the internal and first external loadframes. Inversion of the actuator system relative to the position shownis equally acceptable.

The torsional actuation system concept, which is shown with threetubular actuators 17 in FIG. 7, in general has a plurality of tubulartorsional actuators 17 nested concentrically wherein the adjacentactuators are configured to produce rotational deflections of oppositesign for a signal of a given polarity, and are joined by annular ties 26in a zigzag cross-sectional pattern wherein each tie 26 joins a pair ofadjacent actuators 17, with the tie 26 located at the end of theassembly corresponding to the sign of the deflection of the outermostactuator of the pair, and wherein the free ends of the innermost andoutermost actuators are connected to the second ends of the adjacentload frames at points 9 and 12. The particular sign convention (righthand or left hand) used in the arrangement is not critical but willdetermine the sign of the resultant deflection.

Lastly, an example will be given of sample mounting hardware to enabletesting of the preferred sample in conventional prior art load frames,though the use of such an arrangement will be subject to the limits ofthe particular machine used, with regard to test frequency, etc.

FIG. 8 shows an example of mounting hardware to permit loading thepreferred sample geometry in a conventional (servohydraulic,servoelectric, etc) test machine. For illustration purposes, it isassumed that the prior art test machine is configured with two parallelloading plates 31 for cyclic compression loading 23, but the desiredtest state in the sample is tension. This is accomplished by use of aninternal load frame 2 similar to the load frame extension describedpreviously, but connecting with one of the load plates 31 by directcontact. While the direct contact connection with the loading plates 31is shown between two flat surfaces, the internal load frame mayalternately have a spherical contact surface, to allow for misalignment.An external load frame extension 30 is also utilized, substantiallyenclosing the sample over at least a portion of its length, andextending from a connection interface 6 between the first end of thesample 1 and the first end of the external load frame 30, to a flat-toflat connection with the second load plate 31 at the second end of theexternal load frame extension.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other connectionmethods for different specimen geometries or different test machineconfigurations can be easily devised by one skilled in the art.Therefore, the spirit and scope of the appended claims should not belimited to the description of the preferred versions contained herein.The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

LIST OF REFERENCE SYMBOLS

-   1 Sample-   2 Internal load frame-   3 First external load frame-   4 Guiding interface-   5 Solid state actuator/actuation system acting between internal and    first external load frames-   6 Connecting interface at first end of first external load frame or    external load extension-   7 Connecting interface at first end of internal load frame-   8 Connecting interface at second end of internal load frame-   9 Connecting interface at second end of first external load frame-   10 Second external load frame-   11 Power source-   12 Connecting interface at second end of second external load frame-   13 Connecting interface at first end of second external load frame-   14 Internal load frame extension-   15 Isolating member to protect torsional actuator(s) from axial    loads-   16 Terminus of loading hole in sample-   17 Solid state actuator/actuation system acting between first    external and second external load frames-   18 Furnace/heat source to heat sample for elevated temperature    testing-   19 Insulating members-   20 Crack growing in specimen-   21 Active cooling fluid flow-   22 Load cell-   23 Loads assumed for purpose of illustration-   24 Intermediate member-   25 Tubular axial actuator-   26 Annular connecting tie-   27 Adjustable length connection-   28 First end of actuator system-   29 Second end of actuator system-   30 External load frame extension-   31 Loading platens from prior art test machine-   32 Direct contact interface between a convex spherical surface in a    first member and a flat or matching concave spherical surface in a    second member-   33 Locknut

The invention claimed is:
 1. A sample for fatigue and crack growthtesting, comprising: (a) a length of the material to be testedcomprising: (i) a first and second end; and (ii) an axisymmetric shapeon the exterior over at least a portion of its length; and (b) asubstantially circular hole extending from the first end of the samplealong its longitudinal axis to a depth such that its terminus lies inthe midst of the said axisymmetrically shaped portion, wherein: (i) thehole is concentric with said axisymmetric portion; and (ii) the hole hasa constant diameter along at least a portion of its length in thevicinity of the terminus; and (iii) the region of the terminus of thehole forms a notched shape to initiate and/or grow cracks in thespecimen under cyclic loading; and (c) at least two means of contactinga test machine capable of transmitting loads, wherein: (i) a first meansintroduces load in the vicinity of the terminus of the hole; and (ii) asecond means introduces load at or near the first end of the sample,distributing load in a substantially axisymmetric manner around thesample axis.
 2. The sample according to claim 1, wherein the said firstmeans of contact to a test machine comprises an internal load frame,with at least one member, that extends into the hole, guided by slidingcontact within the hole to form a substantially concentric and coaxialalignment between the hole and the internal load frame, and interfacingwith the hole in the vicinity of its terminus by direct contact betweensubstantially matching flat, spherical, or conical surfaces for thepurpose of applying said load in a direction along the axis of thesample.
 3. The sample according to claim 2, wherein the substantiallymatching flat, spherical, or conical surface for the purpose of axialload introduction comprise a flat bottomed, circular hole terminus withsubstantially sharp corners and a substantially matching flat-endedinternal load frame extension with substantially sharp corners.
 4. Thesample according to claim 1 wherein: (a) the said axisymmetric portionincludes a circular cylindrical shape over a portion of its length; and(b) the said terminus of the hole lies within the said cylindricalportion.
 5. The sample according to claim 4, wherein the ratio of thesaid hole diameter to the said outer diameter of the sample in thevicinity of the hole terminus is less than 0.6.
 6. The sample accordingto claim 1, wherein the said second means of contact to a test machineincludes a threaded connection, or a flange extending radially outwardfrom said first end of the sample, or a combination thereof.
 7. Thesample according to claim 1 further comprising a third means of contactat the second end of the sample for introduction of compression ortorsional loads.
 8. A means for mounting the sample according to claim 2within a conventional load frame configured with two parallel plates forcompressive loading, wherein the said internal load frame extensionprotrudes from the said first end of the sample and interfaces by adirect contact connection to one of the loading plates; and furthercomprising: (a) an external load frame extension with a first and secondend, substantially enclosing the sample over at least a portion of itslength; comprising: (i) means for connecting the first end of the sampleto the first end of the external load frame extension, such as by athreaded or flanged interface; and (ii) a flat-to-flat direct contactconnection between the second end of the external load frame extensionwith the second loading plate.
 9. A method of axial cyclic testing thesample of claim 1 comprising the steps of: (a) mounting the samplebetween two load frames, (b) connecting the load frames to a solid stateactuator system, with the actuator arrangement such that the specimen issubstantially loaded in tension when the actuator system is incompression, (c) tightening a variable length connection in a load trainconsisting of the specimen, actuator system, and load frames connectedtogether until a desired preload is met, with the actuator system incompression, (d) cyclically energizing the solid state actuator systemto create a cyclic load in the sample.